Novoazovsk, Ukraine: Tanks, artillery and infantry have crossed from Russia into a previously unbreached part of eastern Ukraine in recent days, attacking Ukrainian forces and causing panic and wholesale retreat not only in this small border town but also in a wide section of territory, in what Ukrainian and Western military officials described as a stealth invasion.

Ukrainian soldiers wait to move into the town of Mariupol. Photo: AP

The attacks outside Novoazovskand in an area to the north essentially have opened a new, third front in the war in eastern Ukraine between government forces and pro-Russian separatists, along with the fighting outside the cities of Donetsk and Luhansk.

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Exhausted, filthy and dismayed, Ukrainian soldiers staggering out of Novoazovsk for safer territory said on Tuesday, local time, that they were cannon fodder for the forces coming from Russia. As they spoke, tank shells whistled in from the east and exploded nearby.

Some of the retreating Ukrainian soldiers appeared unwilling to fight. The commander of their unit, part of the 9th Brigade from Vinnytsia, in western Ukraine, barked at the men to turn around, to no effect.

Bystanders watch a fire consuming a school in Donetsk. Photo: AFP

"All right," the commander said. "Anybody who refuses to fight, sit apart from the others."

Eleven men did, while the others returned to the city.

Some troops were in a full, chaotic retreat: a busload of them careened past on the highway heading west, purple curtains flapping through windows shot out by gunfire. A Ukrainian military spokesman said on Wednesday the army still controlled Novoazovsk, but that 13 soldiers had died in the fighting.

A resident opens a box with rockets left behind by the Ukrainian army in Starobecheve. Pro-Russian rebels have since taken control of the village. Photo: AFP

The behaviour of the Ukrainian forces corroborated assertions by Western and Ukrainian officials that Russia, despite its strenuous denials, is orchestrating a new counteroffensive to help the besieged separatists of the Donetsk People's Republic, who have been reeling from aggressive Ukrainian military advances in recent weeks.

"Russia is clearly trying to put its finger on the scale to tip things back in favour of its proxies," said a senior US official. "Artillery barrages and other Russian military actions have taken their toll on the Ukrainian military."

The Obama administration, which has placed increasingly punitive economic sanctions on Russia because of the Ukraine crisis, said over the past few days that the Russians had sent new columns of tanks and armour across the border.

A pro-Russian rebel in Donetsk. Photo: AP

"These incursions indicate a Russian-directed counteroffensive is likely under way," US State Department spokeswoman Jen Psaki said on Wednesday.

Ms Psaki also criticised what she called the Russian government's "unwillingness to tell the truth" that its military had sent soldiers as deep as about 50 kilometres inside Ukraine's territory.

She was apparently referring to videos of captured Russian soldiers, distributed by Ukraine's government on Tuesday, that directly challenged President Vladimir Putin's assertions that Russia is a mere bystander in the conflict.

A Ukrainian serviceman shoots during fighting with pro-Russian separatists in the eastern Ukrainian town of Ilovaysk. Photo: Reuters

Russian forces have been trying to help the separatists break the siege of Luhansk and have been fighting to open a corridor to Donetsk from the Ukrainian-Russian border, Western officials say.

To the south, Russia has been backing a separatist push toward the southern town of Mariupol, a major port on the Sea of Azov, according to Western and Ukrainian officials. The Russian aim, one Western official said, was to open a new front that would divert Ukrainian forces from Donetsk and Luhansk and to possibly seize an outlet to the sea in the event that Russia tries to establish a separatist enclave in eastern Ukraine.

Some Western officials fear the move might even be a step in what they suspect is a broader Russian strategy to carve out a land link to Crimea, the strategic Ukrainian peninsula that Russia annexed in March.

Third front: Novoazovsk. Photo: New York Times

The Russian military's use of artillery from locations within Ukraine is of special concern to Western military officials, who say Russian artillery has already been used to shell Ukrainian forces near Luhansk.

Along with the anti-aircraft systems operated by separatists or Russian forces inside Ukraine, the artillery has the potential to alter the balance of power in the struggle for control of eastern Ukraine.

Russia has denied that it has intervened militarily in Ukraine and the separatists have asserted that they are using captured Ukrainian equipment. But US officials say they are confident the artillery in Ukraine's Krasnodon area is Russia's, since Ukrainian forces have not penetrated that deeply into that separatist-controlled region. US officials also say the separatists have no experience in using such weaponry.

"We judge that self-propelled artillery is operated by Russians rather than separatists since no separatist training on this artillery has occurred to date," an Obama administration official said.

The US has photographs that show the Russian artillery moved into Ukraine, its officials say. One photo dated August 21 shows Russian military units moving self-propelled artillery into Ukraine. Another photo, dated August 23, shows the artillery in firing positions in Ukraine.

The Ukrainian retreat from the border area near Novoazovsk came as Mr Putin told his Ukrainian counterpart Petro Poroshenko that the insurgency was an internal matter and the Ukrainian government needed to negotiate a ceasefire.

By coupling the Russian military actions with his political talks with Ukraine in Belarus, Mr Putin appeared to be calculating that Moscow could intervene in eastern Ukraine with conventional Russian forces without risking further Western economic sanctions.

On the highway in Novoazovsk, Sergeant Ihor Sharapov, a soldier with the Ukrainian border patrol unit, said he had seen tanks drive across the border but marked with flags of the Donetsk People's Republic.

The group that attacked the city crossed from Russia, and though some soldiers were convinced they had spent two days fighting the Russians, others said they had no way of knowing who was inside the tanks, or the identities of the infantry who crossed the border and advanced towards the town.

"I tell you they are Russians, but this is what proof I have," said Sergeant Aleksei Panko, holding up his thumb and index finger to form a zero. Sergeant Panko estimated about 60 armoured vehicles crossed near Novoazovsk.